1. Field of the Invention
This invention is directed to a hybrid energy storage system for supplying power to an application with a fluctuating load profile, such as, for example, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, wind energy harvesting equipment and solar energy harvesting equipment.
2. Discussion of Related Art
Recently there has been an increasing interest in environmentally friendly applications such as, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, wind energy harvesting equipment, and solar energy harvesting equipment. These applications typically have fluctuating load profiles that present challenges in the design of energy storage systems for the applications. Conventional approaches for powering these applications have various shortcomings.
Batteries are commonly used for energy storage in the above described applications. However, using batteries as the sole energy source has several disadvantages. For example in an electric vehicle application, in order to approach the performance of a conventional car, the battery should provide the motor with an equivalent or similar power capability as an internal combustion engine. Unfortunately, most available batteries have a relatively low power density. Although there are high power density batteries available, their price is typically much higher than low power density batteries and with the increased power density, thermal management of the battery becomes a challenge. The life of the battery is another major area of concern. In advanced automotive applications, because the load profile varies rapidly according to the road conditions and the driver's behavior, the energy storage system suffers from random charges (e.g. regenerative braking) and discharges (e.g. accelerating), which have a negative effect on the life of the battery. Balancing of a voltage of each cell in a battery system is another problem concerning the battery because, without a balancing system, individual cell voltages will drift apart over time and the voltage capacity of the total pack will decrease quickly during operation, which can result in the failure of the entire battery system. This condition is especially severe when the battery has a long string of cells or the battery is used to do frequent high rate charges and discharges.
To overcome the disadvantages of battery systems, hybrid energy storage systems have been proposed. Hybrid energy storage systems attempt to combine at least two power sources to achieve a better overall performance. The goal of such hybrid systems is generally to take advantage of characteristics of each type of power source, such as, the high energy density of batteries and the high power density and cycle life of ultra-capacitors.
FIG. 1 shows an example of a conventional hybrid energy storage system having an ultra-capacitor/battery configuration. In this configuration, a battery is directly connected to a DC bus and an ultra-capacitor is connected to the DC bus via a bi-directional DC/DC converter. This configuration allows the ultra-capacitor to be used over a wide voltage range and a nominal voltage of the ultra-capacitor can be lower. Connecting the battery directly to the DC bus allows a DC bus voltage to be maintained relatively constant. However, this configuration also has disadvantages. For example, the energy generated by an application, such as regenerative braking, cannot be effectively controlled to be absorbed by the ultra-capacitor and instead the generated energy is directed to the battery, thereby shortening the life of the battery. Additionally, to properly use the power of the ultra-capacitor, the bi-directional DC/DC converter should be of an equivalent size. A large bi-directional DC/DC converter can be expensive and has thermal management issues that must be addressed.
FIG. 2 shows another conventional hybrid energy storage system having a battery/ultra-capacitor configuration. In this configuration, the ultra-capacitor is directly connected to the DC bus and the battery is connected to the DC bus via a bi-directional DC/DC converter. In this configuration, a voltage of the battery can be maintained lower and the ultra-capacitor works as a low pass filter. This configuration allows a regenerated energy from the application to be directed to the ultra-capacitor preserving the life of the battery. However, this configuration limits the working range of the ultra-capacitor.
A third conventional configuration, shown in FIG. 3, includes a second bi-directional DC/DC converter between an ultra-capacitor and a DC bus. This forms a cascaded converter topology. This configuration improves the working range of the ultra-capacitor but requires the second bi-directional DC/DC converter. Disadvantages of this configuration include additional expense for a second converter and reliability issues.
FIG. 4 shows a conventional hybrid energy storage system having a multiple converter configuration. A disadvantage of this configuration is that two converters are required.